The Continental Cough and Prolonged Single Ignition Operation

The Continental Cough and Prolonged Single Ignition Operation

There is an old adage that applies to life in general and to aviation in particular that states that if you pay attention you will always have something new to learn. This fact of life was recently brought to my attention when I gained some new knowledge of the operation of the Continental W670, 220 h.p. engine. I have over 30 years experience and between 3,000 – 4,000 hours of flying time amassed in Stearmans, most of it behind the Continental W670 engine. The specific information that I just recently learned certainly was new to me and perhaps there are many other Stearman pilots who also are not aware of some of the details pertaining to the “Continental Cough”.

The genesis of this episode of “I Learned About Flying From That” began about three years ago when I had the engine on my Stearman overhauled. The new engine ran fine initially, but the rings never did seat in the chrome overhauled cylinders and the oil consumption was over 2 qts./hour. This high oil consumption was visible to the casual observer due to the dark black smoke which streamed out of the exhaust and the black sooty film that covered the right side of the airplane. We put 77 hours on it the first year and that winter we removed all the cylinders and sent them back in to be overhauled and chromed again.

The next year’s flying saw us put another 119 hours on the engine and once again we encountered the same problem—the rings refused to seat and the oil consumption remained at over 2 qts./hour. In addition, the engine began to exhibit a roughness while running and gave a large r.p.m. drop while at high power on a single magneto as well as during the normal engine run-up magneto check. We tried to find a solution to the rough running and eventually had the carburetor overhauled, both magnetos rebuilt, new P-lead wiring installed from the ignition switch to the mags and even put in a new ignition switch. All this was to no avail as the engine continued to cough and snort on a magneto check on run-up and would still misfire and intermittently quit running in flight whenever we switched to check its operation on one magneto at cruise power. I finally became convinced that the problem lay in the ignition harness, but a check with a lead tester showed no problems with the wires. Needless to say, we were very frustrated with all this and basically had lost all confidence in this engine. Consequently, my son, Mike, and I got to the point that we were almost afraid to fly the airplane out of gliding range of the airport. And as every Stearman pilot knows, that’s not very far!

Well, the next winter I decided to try to fix it one more time and we removed the cylinders again. But this time I vowed I would not send them back to the company that had already overhauled and chromed them twice. I had heard a lot of very good reports about the engines that Don Sanders of Sanders Airmotive, Inc. in Mustang, Oklahoma had overhauled. So I shipped the cylinders to him. He eventually reported back to me that the cylinders had been chromed with a choke bore instead of a straight bore and that the pistons were slapping around inside the cylinders and it was no wonder the rings couldn’t seat. He said that he could re-chrome and overhaul my cylinders, but instead I decided to accept his offer of brand new surplus cylinders with matched pistons and rings.

We installed the new cylinders on the engine and the rings seated in right away and the oil consumption was normal. But the engine continued to run rough and had a definite “pounding” to it and still had the ignition problems just as before. Needless to say, we had just about reached our wits end.

Now about this time (spring of 2000), Mike and I became acquainted with Scott (SRA 3544) and Aaron (SRA 3511) Marshall of Waukesha, Wisconsin who had just purchased a Stearman in the late fall of 1999. Aaron had gotten his check-out completed with John Lohmar (SRA 2188) shortly after buying the airplane and Mike and I had the pleasure of flying with Scott to finish his check-out. Well, their Stearman had a brand new Don Sanders overhauled Continental 220 engine that they had purchased from John Lohmar through his company, Heritage Airplane, at Creve Coeur Airport, near St. Louis, Missouri. It ran like a dream! It was smooth as a sewing machine and the oil consumption was negligible. All the time we flew together, I just sat there in the front cockpit lusting after his engine! After completing the check out, Scott asked me how much he owed for the dual flight instruction. I told him, “Nothing! I just want your engine.”

That Stearman check-out with Scott proved to be the most expensive one I had ever done because I just had to have a new Don Sanders engine for my Stearman. I knew that John Lohmar happened to have another one of Don’s freshly overhauled engines stored on a pallet in his shop, so I called him and made the purchase. Don’s engines come with a brand new Skytronics shielded ignition harness installed and I then proceeded to do one of those stupid things that pilots do upon occasion and asked John if he could remove that and install an original type unshielded ignition harness on it for me instead. That would prove to be another one of those decisions that turns around and bites you in the rear and eventually would prove that I had just shot myself in the foot.

John Lohmar asked, “Why do you want to do that? These ignition harnesses look almost like the original one and work great. You sure you want to do this?” I assured him that I did and explained my reasoning. On our previous engine I originally installed a shielded ignition harness because for a while I labored under the misconception that I might later install a radio and transponder in my bare basic Stearman. I had a lot of problems with this harness, mainly with moisture collecting in the spark plugs and ignition wire cigarettes causing arching and intermittent engine misfiring. Eventually we removed it and installed an original styled non-shielded harness. Mike and I both abhor the thought of all that electronic gear (radios, transponders and GPSs, etc.) cluttering up the cockpit of our Stearman, so my decision to have John’s shop install another unshielded harness complied with our prejudices. I know that many Stearman pilots are required to have all this stuff in their airplanes due to airspace and air traffic control requirements at their home airports. Fortunately for us and our warped viewpoint, we fly our Stearman out of a grass field in uncontrolled airspace.

The new engine ran great. We were ecstatic to finally have a fine running, dependable engine on our Stearman. With about 18 hours on it flying in the local area we departed for Galesburg and the 29th National Stearman Fly In. Several days of enjoyable flying were had until Mike landed one day and reported that the engine had quit on him while on downwind leg in the pattern. It had been just a momentary interruption, but still definitely had quit for just a second or two. “Oh no, I thought. Here we go again.”

Subsequent inspection showed that the ignition wire for the rear sparkplug on cylinder number three had made contact with the exhaust stack and was burned about half-way through. Further inspection revealed that the insulation on almost all the other ignition wires was cracked and deteriorating. So much for our “new surplus” ignition harness. (Remember—“new surplus” is still WW II vintage and around 60 years old!)

I was aware that Don Sanders was planning to attend the Stearman Fly In for a couple of days so I called him and fortunately caught him before he left home. He packed in some new ignition wire and upon his arrival in Galesburg installed a new ignition harness on the engine. A quick check of the engine and a test flight proved it to be running normally again. The point of the story up to this time relative to the new ignition harness is: “If it ain’t broke, don’t fix it.”

The next day while Mike again was flying the airplane, he reported that the engine was running rough and misfiring whenever he ran it on one mag—it didn’t make any difference which one—while checking it on the ground. However though, it appeared to be normal in flight. It seems as though whenever anything goes wrong with our Stearman, Mike is always the one flying it. For that reason I have toyed with the idea of rescinding his flying privileges, but I guess I really wouldn’t do that.

The Continental W670 Overhaul Manual states that the magneto check should be made at 1200 – 1500 r.p.m. and the WW II U.S. Navy training films said that the run-up magneto check was to be done at 1400 r.p.m. So for that reason, we have always done the mag check at 1400 r.p.m. Well, with all the problems we had had with our previous engine we had begun checking the magnetos individually for an extended period of time during the run-up, as well as in flight. This paranoia continued on with our new engine. We just couldn’t seem to be content to do a normal check. Whenever we did a normal magneto check at 1400 r.p.m., the r.p.m. would drop about 50 r.p.m. But when we left it on either magneto individually for more than about ten seconds the r.p.m. began to spool down to 1000 r.p.m. or less and the engine began snorting and backfiring like it had an induction leak and belched black smoke out the exhaust stack. Once the mags were switched back to “Both”, it ran normally again. Once more we were stupefied and disappointed that we were having almost the same problem with our brand new engine as we had had with our previous engine.

After the Fly In was over the mechanics at Jet-Air, Inc. in Galesburg did a very thorough 25 hour inspection on the engine and they found that the valve clearances on two cylinders were way out of limits. Two valves had tightened down and would not open, while the other two valves had loosened up and would not close. No wonder it wouldn’t run right! They re-set all the valve clearances and also re-adjusted the timing on both magnetos. They reported back to me that the engine ran great. Both of these re-adjustments are quite normal on a freshly overhauled engine. The Continental W670 Overhaul Manual recommends that the valve clearances be re-set every 100 hours, but most mechanics recommend that they initially be re-adjusted 25 hours after the overhaul. They also suggest that the magneto timing should also be re-checked after 25 hours, as it is possible for the mags to slip slightly during the first 25 hours. Our engine had about 23 hours on it, so these adjustments would be considered as normal maintenance for a new engine.

Mike and I returned to Galesburg prepared to fly good “Ole #17” home. The engine was just as advertised, it ran great and very smoothly and we were satisfied that the valve clearance variance was what had caused the problem and everything was fine. We did our normal run-up and it again showed about a 50 r.p.m. magneto drop. But, with paranoia still reigning in our hearts, we proceeded to do a single-magneto run for an extended time and immediately the engine started spooling down, snorting, barking, backfiring and belching black smoke out of the exhaust stack. Switching the mags back to “Both” eliminated the problem. However, the engine ran fine on one magneto at high power settings. Once again we were baffled and quite concerned with what could possibly be wrong with this new engine.

Well, several telephone conversations ensued between us at Galesburg and Don Sanders trying to figure out what possibly could be causing this problem. Numerous different solutions were put forward: replace the mags; replace the coils and condensers in the mags; replace the carburetor; install new spark plugs, check the ignition wires, etc. Finally, Don called us back and asked, “What exact r.p.m. are you running at when you check the mags and leave it on either L or R for a long time?” I replied, “1400.” Don in turn replied, “Well, that’s it! That’s a perfectly normal indication at that r.p.m. Don’t worry about it, just run it up to a bit higher r.p.m. and it will be normal.”

Well, it’s certainly been a long drawn out account to finally get to the crux of this story and to the point of “The Continental Cough and Prolonged Single Ignition Operation”. But it was important to lay the background of our engine experiences over the last three years to show why we did what we did and how it led up to this new-to-us insight into the operation of the Continental W670 engine. As Paul Harvey would say, “Now, The Rest of the Story” as explained to me by Don Sanders.

The Continental W670 engine has a weak running area, generally between around 1100 r.p.m. to 1500 r.p.m. This is due to a combination of several features of the general design of the engine and most specifically with the carburetion system. The engine originally was equipped with a Stromberg NA-R6 carburetor and it tended to cough and hesitate as the engine throttle was advanced to increase the r.p.m. of the engine. In an attempt to correct this problem the engine next had the NA-R6D carburetor installed. This was an improvement, but the “Continental Cough” continued to be present. On November 25, 1942 in order to improve the operation of the Stromberg NA-R6D carburetor, the Stromberg Aircraft Carburetor Service, Bendix Products Division, of South Bend, Indiana issued Service Bulletin #59. This service bulletin recommended drilling a 1/8 inch hole on a 15º angle to create a vent channel between the well above the mixture control needle and the float bowl and also to replace the float assembly 390260 with a new assembly 383565. After this service bulletin was complied with, the carburetor was to be redesignated as a NA-R6G. The NA-R6G carb pretty much solved the “cough” problem, but not entirely. The Continental W670 Engine Overhaul Manual specifies that the NA-R6D was to be installed on W670-A, 6N, K and M model engines. The NA-R6G carb is to be used on the –16, -17, -23 and –24 model engines. But many NA-R6G carbs are now commonly being used on the other dash model engines such as the –6A and –6N. As most Stearman pilots are aware, the Continental W670 often still will exhibit the “cough” as the throttle is advanced during take-off. This is normal.

As to the problem we encountered with our engines barely running while we checked the operation on one magneto at 1400 r.p.m. Don Sanders indicated that that also is a quite normal indication and is a part of the well-known “Continental Cough” syndrome. While doing a normal magneto check at up to about 1500 r.p.m. the engine will run normally and you can expect a normal r.p.m. drop of 50-75 r.p.m. However, if you leave the magneto switch on just one mag for more than about ten seconds (as we had been doing) the engine will spool down to as low as 1000 r.p.m., cough, snort, backfire, belch black smoke out the exhaust and sound like it’s sucking air. Don says that this is normal in this mid-range of r.p.m. The engine just will not run properly on one magneto for an extended period of time in this r.p.m. range. The engine will give a normal magneto drop at 1400-1500 r.p.m. with the mag switch moved to each magneto position for a few seconds. With the engine r.p.m. above 1500, the engine will run normally on one magneto only.

Don Sanders states that he runs his newly overhauled engines on a test stand at pre-determined r.p.m. of 1000, 1200, 1300, 1400, etc., for specific time periods during the break-in. He has noted that the engines will demonstrate the “Continental Cough” occasionally during the break-in runs at these lower r.p.m. even on both magnetos.

Most all of us Stearman pilots have long been aware of the “Continental Cough” that occurs periodically when the throttle is advanced to take-off power. But I was not fully aware of the cause of it and that it is most susceptible in the mid-r.p.m. range of the engine. Our episode of checking the engine for long periods of time on one magneto in this mid-range brought us to new knowledge of the “Continental Cough” and a better understanding of the operation of the engine.

In conclusion, the “Continental Cough” is a normal condition exhibited by the W670 engine as the throttle is advanced through the mid-range of about 1100-1500 r.p.m. The engine will not run normally in this range on only one magneto and occasionally will demonstrate the “cough” even on both magnetos. Above 1500 r.p.m. the engine will run normally and will run for an indefinite time on one magneto. This of course, is the reason for dual magnetos.

By Tom Lowe|
2001-05-16T17:42:31+00:00 May 16th, 2001|Flying-Wire|Comments Off on The Continental Cough and Prolonged Single Ignition Operation

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